1. Field of the Invention
The present invention relates to a system and method for controlling a position in a controlled object in a mechanical equipment, such as a numerical controlled (NC) machine tool.
2. Description of Related Art
A position control in a controlled object, such as a work table and a work cutter in a NC machine tool is, in general, done by controlling a rotating amount of a servomotor connected to the controlled object via a transmission mechanism, which is, for example, constructed by a rack-and-pinion mechanism or a ball screw-and-nut mechanism.
A positional control in a controlled object, of a semi-closed-loop type, a full-closed-loop type and a hybrid control type is known.
In the semi-closed-loop positional control, a servo control system is constructed such that a detection of a rotating position of a servomotor is done by, for example, using a detector such as a rotary encoder. The detected rotating position of the servomotor is, then, transformed into a position of the controlled object. The transformed rotating position is, finally, subjected to a feedback to a positional command for controlling a rotating amount of the servomotor.
In the full-closed-loop positional control, a servo control system is constructed such that a linear type measuring scale is, for example, directly connected to a controlled object such as a table. The position of the controlled object as detected by the linear type measuring scale is subjected to a feedback to a positional command for controlling the rotating amount of the servomotor.
In the hybrid control positional control, a servo control system is constructed such that a rotating position of a servomotor and a position of the controlled object are both detected. A rotating position of the servomotor transformed to the position of the controlled object is subjected to a feedback to the positional command while a difference between a position of the controlled object and the transformed rotating position of the servomotor is, first, subjected to a filtering by a first order lag filter and is, then, subjected to a feedback to the positional command.
When a non-linear characteristic such as a backlash or a friction exists in a transmission mechanism arranged between the servomotor and the controlled object, a phenomenon is generated, wherein a switching of the direction of the movement of the controlled object causes the servomotor to be moved, while the controlled object is maintained to be unmoved. Such a phenomenon is called as a lost motion, by which the controlled object is not able to respond quickly to the positional command.
A method for correcting the tracking error is known, which is called as a back lash correction, wherein, upon a detection of the switching in the direction of the movement in the positional command, a correction in the control command to the servomotor is done, so that the lost motion is quickly removed, thereby restricting the response error from the positional command in the controlled object.
In the above mentioned semi-closed-loop positional control, the position of the controlled object is indirectly obtained from the rotating position of the servomotor, i.e., a direct control is not done. As a result, a backlash correction does not cause any offset to be generated in the controlled object.
Contrary to this, in the above mentioned full-closed-loop positional control, an execution of the above mentioned a backlash correction during a reversal in a feedback value as obtained by the linear scale causes an offset to be generated in the position of the controlled object due to the fact that a positional information of the controlled object is also corrected.
As far as the hybrid control type is concerned, not only a feedback value from the linear scale but also a feed back value of a servomotor are used. Therefore, an occurrence of the offset in the controlled position of the controlled object is prevented by executing a correction of the feedback value.
However, in the hybrid control positional control, a control is done in such a manner that a coincidence is obtained between the target position and the detected position of the linear scale. As a result, even a small change of a feedback value of the position of the controlled object as obtained by the linear scale in a direction opposite to the direction of occurrence of the lost motion causes the servomotor to move the controlled object beyond the range of the lost motion irrespective of a fact that the positional command is unchanged. A stoppage of the controlled object is obtained when the coincidence between the feedback value of the position of the controlled object and the positional command is obtained, thereby canceling the lost motion. A detection of the reversal in the moved direction of the positional command from this condition causes, however, a phenomenon to be generated, wherein a momental deviation of the controlled object from the target position is generated due to the operation of the backlash correction device. In a system where the control object is moved along an arc shaped pass by employing a orthogonal double axis control, the above-mentioned phenomenon may cause a difficulty to be occurred such that a trajectory of the controlled object is deviated inwardly from the desired arc shaped pass at so-called quadrant switching points.
An object of the present invention is to provide a position control system and method of a type wherein a position of a controlled object is directly detected and is subjected to a feedback operation for controlling the position of the controlled object, capable of overcoming the above mentioned difficulties in the prior art.
Another object of the present invention is to provide a position control system and method of the above type, capable of restricting a tracking error in the controlled object as generated upon a reversal in the direction of the feed of the controlled object.
Still another object the present invention is to provide a position control system and method of the above type, capable of preventing any offset from being generated in controlled position of the controlled object.
According to the first aspect of the present invention, there is provided a positional control system comprising: a controlled object: driving means connected to the controlled object; setting means for setting a variable target or desired position of the controlled object; a control command generating means for generating a control command in accordance with said variable target position; a position sensor arranged on said controlled object for detecting a position of the controlled object; a deviation generating means for generating a deviation from said control command and a detected position of the controlled object; a serve control means for generating, in accordance with the deviation, a manipulated variable issued to said driving means for causing said controlled object to follow said variable target position; and, a tracking error correction means operated in a period which is commenced when a direction in the feed in the variable target position is reversed and when the controlled objected is stopped and which is ceased when the movement of the controlled object is re-started, for correcting the deviation, so that a tracking error of said controlled object with respect to said variable target position as generated by said reversal in the direction of the feed is restricted.
Preferably, said tracking error correction means comprise: a reversal detector part detecting a reversal in direction of feed; a movement/stoppage detector part for detecting if the controlled object is moved or stopped; and, a tracking error correcting part for issuing a correcting amount for correcting said deviation in accordance with signals from said reversal detector and said movement/stoppage detector.
Preferably, said tracking error correcting part holds, in accordance with the direction of the reverse of the feed, a first and second correction amounts of opposed signs of the same absolute value.
Preferably, said movement/stoppage detector part detects, based on the signal from said position sensor, detects if the controlled object is moved or stopped.
According to the second aspect of the present invention, a position control method is also provided, comprising the steps of: providing a controlled object; providing driving means connected to the controlled object; setting a variable target or desired position of the controlled object; generating control command in accordance with said variable target position; providing a position sensor in a controlled object; detecting a position of the controlled object by said position sensor; generating a deviation from said control command and a detected position of the controlled object; effecting a servo-control by generating, in accordance with the deviation, a manipulated variable issued to said driving means for causing said controlled object to follow said variable target position; and, correcting said deviation during a period which is commenced when a direction in the feed in the variable target position is reversed and when the controlled objected is stopped and which is ceased when the movement of the controlled object is re-started, so that a tracking error of said controlled object with respect to said variable target position as generated by said reversal in the direction of the feed is restricted.
In the system or method in the first aspect of the present invention, the driving means is operated in accordance with the variable target or desired position, so that the controlled object connected to the drive means is controlled to the variable target position. A backlash between the drive means and the controlled object and a non-linearity such as a friction can not be avoided, which may causes a lost motion to be generated, by which, irrespective of a change in the direction of the operation of the driving means, the controlled object keeps its stopped condition. According to the present invention, when the direction in feed of the variable target position and a stoppage of the controlled object are detected, a correction of a control deviation is done only at a period where the controlled object is stopped, thereby restricting a tracking error of the controlled object. Furthermore, according to the present invention, the correction is done during the stoppage of the controlled object. As a result, any change in the position sensor on the controlled object does not occur, so that any information as to a change in the detected position is not included in the control deviation prior to the execution of the correction. As a result, an information of the detected position of the controlled object is not varied irrespective of the correction of the control deviation. Thus, any offset in the controlled position of the controlled object is not generated when the movement of the controlled object is re-restarted.
According to a third aspect of the present invention, there is provided a positional control system comprising: a controlled object; a driving means connected to the controlled object; a setting means for setting a variable target or desired position of the controlled object; a control command generating means for generating a control command in accordance with said variable target position; a first position sensor on said controlled object for detecting a position of the controlled object; a second position sensor arranged on said driving means for detecting a driving position of the driving means; a deviation generating means for generating control deviation of the controlled object from said control command, the detected position of the controlled object and detected driving position of the driving means; a serve control means for generating, in accordance with the control deviation, a manipulated variable issued to said driving means for causing said controlled object to follow said variable target position; and, a tracking error correction means operated in a period which is commenced when a direction in the feed in the variable target position is reversed and when the controlled objected is stopped and which is ceased when the movement of the controlled object is re-started, for modifying a correction amount for correcting said detected driving position, so that a tracking error of said controlled object with respect to said variable target position as generated by said reversal in the direction of the feed is restricted.
Preferably, said tracking error correction means comprise: a reversal detector part detecting a reversal in direction of feed; a movement/stoppage detector part for detecting if the controlled object is moved or stopped; and, a tracking error correcting part for increasing or decreasing a correcting amount for correcting said detected driving position in accordance with signals from said reversal detector part and said movement/stoppage detector part.
Preferably, said tracking error correcting part effects a periodical modification of a predetermined amount of the correcting amount for correcting the detected driving position when a reversal in the direction of the feed is detected by the reversal detecting part as well as when the stoppage of the controlled object is detected by said movement/stoppage detector part, said modification being ceased when a re-start of the controlled object is detected by said movement/stoppage detector part.
Preferably, said movement/stoppage detector part effects, based on the signal from said position sensor, detection if the controlled object is moved or stopped.
According to the fourth aspect of the present invention, there is provided a position control method is also provided, comprising the steps of: providing a controlled object; providing driving means connected to the controlled object; setting a variable target or desired position of the controlled object; generating control command in accordance with said variable target position; providing a position sensor in a controlled object; detecting a position of the controlled object by said position sensor; providing a position sensor in the driving means; detecting a driving position of the driving means by the driving position sensor; generating a deviation from said control command and a detected driving position of the driving means; effecting a servo-control by generating, in accordance with the deviation, a manipulated variable issued to said driving means for causing said controlled object to follow said variable target position; and, modifying correcting amount for correcting the detected driving position during a period which is commenced when a direction in the feed in the variable target position is reversed and when the controlled objected is stopped and which is ceased when the movement of the controlled object is re-started, so that a tracking error of said controlled object with respect to said variable target position as generated by said reverse in the direction of the feed is restricted.
In the second aspect of the present invention, the driving means is operated in accordance with the variable target or desired position, so that the controlled object connected to the driving means is controlled to the variable target position. A reversal in the feed due to the change in the variable target position may cause the lost motion to be generated. However, according to the this aspect of the invention, when a reversal in the feed in the variable target position and the stoppage of the controlled object is detected, a correcting amount for correcting the detected driving position is modified, so that the tracking error in the controlled object is restricted. Furthermore, in this aspect of the present invention, a correction of the detected driving position is done and the corrected amount of modified value is always used for correcting the detected driving position. Thus, any modification of an information in the position of the controlled object does not occur, so that any generation of offset in the controlled position of the controlled object is prevented.